A membrane is a thin, film-like structure that can be used to separate two fluids. In a filtration module, it acts as a selective barrier, allowing some particles or chemicals to pass through, but not others. During membrane filtration, contaminants in the feed water are accumulated on the membrane surface when the permeate passes through the membrane, which leads to an increase of membrane filtration resistance. This is referred to as membrane fouling, and is a major factor for consideration in designing membrane bioreactors (MBRs).
Surface hydrodynamic shear stresses can remove foulants from the membrane surface, thereby alleviating the problem of membrane fouling. Surface shear stress can be generated by relative movement between the membrane and the adjacent fluid. Currently, air sparging is used extensively for this objective in aerobic MBRs (AMBRs). Similarly, biogas sparging is used in anaerobic MBRs (AnMBRs) to remove membrane foulants. However, the flux improvement by air or biogas sparging can be limited (Xia et al. 2013) and its energy consumption can be as high as 70% of the total cost (Judd 2006; Drews 2010). In addition, biogas sparging for AnMBRs poses a critical operational issue since the biogas production can be unstable, and the increase of biogas concentration inside the fluid in the reactor may decrease the anaerobic digestion rates and thus reduce the production of biogas for energy recovery.